This code was weird in the first place. I suspect my change will break something else
(probably the appcode that needed it). ssh_poll_ctx_free is not a good
place to send exception callbacks imho.
(cherry picked from commit b5351f2809)
Functions to mesure elapsed time before and after a serie of
calls. Introduces a dependancy to clock_gettime() and librt,
hope this doesn't break anything. Porting to gettimeofday() should
not be too hard.
(cherry picked from commit 59f7647cd9)
This also fixes error handling in ssh_poll_ctx_dopoll() and
ssh_handle_packets(), so it won't loop forever on an actual timeout.
(cherry picked from commit 671a982739)
